This application claims the benefit of U.S. Provisional Patent Application No. 60/331,349 filed Jul. 31, 2001, and U.S. Provisional Application No. 60/342,657 filed Dec. 20, 2001, both of which applications are incorporated herein by reference in their entireties.
1. Field of the Invention
The present invention generally relates to processes for impregnating porous parts and, more specifically, to processes for impregnating porous parts with an impregnant that is not water soluble and curing the impregnated parts in a water bath.
2. Description of the Related Art
Impregnation of porous parts is a common technique employed in a variety of industries for a variety of reasons. Stone, brick, ceramic, wood, polymer, aggregate, cermet, and porous metal parts, for example, are commonly impregnated. Typically, a sealant is impregnated into the part because the porosity is undesirable in the intended end use of the part. In some applications, it is only necessary to seal the pores on the surface of the part. In other applications, thorough impregnation of the part is necessary. Further, in certain applications it may be possible to over-impregnate a part, so careful control of the level of impregnation is required.
By way of example, fuel cells, including solid polymer electrolyte fuel cells, utilize initially porous components such as separator plates. Separator plates are commonly made from graphite, graphitized carbon or carbon-resin composites.
For example, expanded graphite sheets, such as the material available from Graftech Inc. (Cleveland, Ohio, U.S.A.) under the tradename GRAFOIL, may be used to form separator plates for fuel cells. Expanded graphite sheets are useful in this regard because they are relatively light, flexible and amenable to low-cost manufacturing methods, such as embossing.
Nonetheless, separator plates made from expanded graphite sheets are typically impregnated in order to achieve the desired levels of impermeability and mechanical stability (that is, structural strength and hardness). After impregnation the separator plates are substantially impermeable to the fluid reactants and/or coolants used in the fuel cell or fuel cell stack, mechanically stable and electrically conductive. Known impregnants suitable for such purposes include phenols, epoxies, melamines, furans, and acrylics, such as methacrylates, for example.
It is important that such plates be sufficiently impregnated to meet performance requirements. At the same time, it is possible to over-impregnate the plates, resulting in degradation or loss of desired structural and/or functional properties.
In addition, it is generally undesirable to have residual cured impregnant left on the surface of the impregnated plates. The presence of impregnant deposits on the surface of the cured plate can: affect the electrical conductivity of the plate; interfere with electrical contact between fuel cell components in the assembled cell/stack; be detrimental insofar as thickness tolerances are concerned; and, may also interfere with the function of surface features on the plate. Accordingly, impregnation process control is an important aspect of separator plate manufacture.
In typical industrial processes, curing of the impregnated parts is accomplished by dipping the parts in a hot water bath after washing and rinsing. The washing, rinsing and curing steps can occur in the same vessel. In applications where the impregnant contributes to desired characteristics such as the mechanical stability, and in particular the surface hardness, of the impregnated parts, the typical industrial process is undesirable.
Commercially available impregnants typically include surfactants or other solubilizing agents to make them water soluble. Such impregnants are made water soluble to facilitate washing and rinsing of the impregnated parts. Thus, the washing and rinsing steps are able to remove excess impregnant from the surface of the parts before curing. But, because the impregnant is water soluble, washing and rinsing also removes some impregnant from the pores near the surface of the part. This problem is further exacerbated by the hot water curing, as more impregnant is removed from the part during the curing process. As a result, insufficient impregnant is left in the surface pores of the part(s) to provide the requisite surface hardness. The lack of sufficient impregnant in the surface pores of the part(s) may also lead to sealing problems. The loss of impregnant is particularly problematic with thin impregnated parts where the surface-to-volume ratio is relatively high. Indeed, prior efforts to cure impregnated expanded graphite sheets using a hot water curing process have failed to achieve fuel cell separator plates having the desired levels of impermeability, mechanical stability and/or electrical conductivity.
Accordingly, there remains a need for improved methods for impregnating porous parts, such as fuel cell separator plates. The present invention fulfills this need and provides further related advantages.
In brief, the invention is directed to a process for impregnating a porous part. In one embodiment, the process comprises impregnating the part with an impregnant that is not water soluble, and curing the impregnated part in a water bath.
In another embodiment, the process comprises impregnating a part with an impregnant that is not water soluble; washing the impregnated part in a washing solution; and then curing the impregnated part in a water bath.
The washing solution may comprise water and a surfactant, or other polar solvents, such as acetone, lower aliphatic alcohols (in particular, C1-C4 alcohols), and miscible mixtures thereof (including water-alcohol mixtures). Alternatively, the washing solution may comprise a non-polar solvent, such as hydrotreated heavy naptha.
In another embodiment, a process for impregnating expanded graphite sheet is provided. The embodiment of the present process comprises impregnating the sheet with a heat-curable impregnant; washing the impregnated sheet in a washing solution; and curing the washed sheet in a water bath at a curing temperature, wherein the impregnant is not water soluble.
These and other aspects of the invention will be apparent upon reference to the attached figures and following detailed description.